SC1565I5T-1.8_00 データシートの表示（PDF） - Semtech Corporation

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SC1565I5T-1.8_00

VERY LOW DROPOUT 1.5 AMP REGULATOR WITH ENABLE

Semtech Corporation 

VERY LOW DROPOUT 1.5 AMP

REGULATOR WITH ENABLE

SC1565

October 3, 2000

TYPICAL CHARACTERISTICS (Cont.)

Quiescent Current vs.

Junction Temperature

600

VIN = 3.3V

590

580

570

560

550

540

530

520

510

500

-50 -25

0

25

50

75

TJ (°C)

100 125 150

APPLICATIONS INFORMATION

Introduction

The SC1565 is intended for applications such as

graphics cards where high current capability and very

low dropout voltage are required. It provides a very

simple, low cost solution that uses very little pcb real

estate. Additional features include an enable pin to al-

low for a very low power consumption standby mode,

and a fully adjustable output.

Component Selection

Input capacitor - a 4.7µF ceramic capacitor is recom-

mended. This allows for the device being some dis-

tance from any bulk capacitance on the rail. Addition-

ally, input droop due to load transients is reduced, im-

proving load transient response. Additional capaci-

tance may be added if required by the application.

Output capacitor - a minimum bulk capacitance of

10µF, along with a 0.1µF ceramic decoupling capacitor

is recommended. Increasing the bulk capacitance will

improve the overall transient response. The use of

multiple lower value ceramic capacitors in parallel to

achieve the desired bulk capacitance will not cause

stability issues. Although designed for use with ce-

ramic output capacitors, the SC1565 is extremely toler-

ant of output capacitor ESR values and thus will also

work comfortably with tantalum output capacitors.

External voltage selection resistors - the use of 1% re-

sistors, and designing for a current flow ≥ 10µA is rec-

ommended to ensure a well regulated output (thus R2

≤ 120kΩ).

Off-State Quiescent Current vs.

Junction Temperature

10

VIN = 5.5V

9

8

7

6

5

4

3

2

1

0

-50 -25

0

25

50

75

100 125 150

TJ (°C)

Thermal Considerations

The power dissipation in the SC1565 is approximately

equal to the product of the output current and the input

to output voltage differential:

PD ≈ (VIN − VOUT ) • IO

The absolute worst-case dissipation is given by:

( ) PD (MAX ) = VIN (MAX ) − VOUT (MIN ) • IO (MAX ) + VIN (MAX ) • IQ(MAX )

For a typical scenario, VIN = 3.3V ± 5%, VOUT = 2.8V

and IO = 1.5A, therefore:

VIN(MAX) = 3.465V, VOUT(MIN) = 2.744V and

IQ(MAX) = 1.75mA,

Thus PD(MAX) = 1.09W.

Using this figure, and assuming TA(MAX) = 70°C, we can

calculate the maximum thermal impedance allowable

to maintain TJ ≤ 150°C:

( ) TJ(MAX ) − TA(MAX) (150 − 70)

R = TH(J− A )(MAX )

PD(MAX )

= 1.09 = 73.4° C / W

This should be achievable for the SO-8 package using

pcb copper area to aid in conducting the heat away,

such as one square inch of copper connected to the

ground pins of the device. The SOT-223 and TO-220

packages would not require heatsinking. Internal

ground/power planes and air flow will also assist in re-

moving heat. For higher ambient temperatures it may

be necessary to use additional copper area.

© 2000 SEMTECH CORP.

7

652 MITCHELL ROAD NEWBURY PARK CA 91320

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